This invention relates to electric power inverters for converting direct current (d-c) to polyphase alternating current (a-c), and more particularly it relates to improvements in the old and well known current-fed "third harmonic" auxiliary impulse commutated inverters. The principles of commutation and a typical practical application of such inverters were described in a technical paper entitled: "Analysis of a Novel Forced-Commutation Starting Scheme for a Load-Commutated Synchronous Motor Drive," which paper was presented by R. L. Steigerwald and T. A. Lipo at the IEEE/IAS annual meeting held in Los Angeles, Calif. on Oct. 2-4, 1977. The Steigerwald and Lipo paper was reprinted in IEEE TRANS. Vol. IA-15, No. 1, January/Febuary 1979, pgs. 14-24, and it is expressly incorporated herein by reference.
In essence, a third harmonic auxiliary impulse commutated inverter comprises six main unidirectional conduction controllable electric valves, such as thyristors, that are interconnected in pairs of series aiding, alternately conducting valves to form a conventional 3-phase, double-way, 6-pulse bridge between a pair of d-c terminals and a set of three a-c terminals. The d-c terminals of the bridge are adapted to be connected to a suitable source of relatively smooth direct current. A large, multicell, heavy duty electric storage battery. is a suitable current source, as is the combination of an electric power rectifier to which an alternating voltage source is connected and a current smoothing reactor or choke in the d-c link between the d-c terminals of the rectifier and inverter, respectively. The a-c terminals of the aforesaid bridge are respectively connected to the different phases of a 3-phase electric load circuit which typically comprises star-connected 3-phase stator windings of a dynamoelectric machine such as a large synchronous motor.
To supply the load circuit with 3-phase alternating current, the six main valves of the inverter are cyclically turned on (i.e., rendered conductive) in a predetermined sequence in response to a family of "firing" signals (gate pulses) that are periodically generated in a prescribed pattern and at desired moments of time by associated control means. To periodically turn off the main valves by forced commutation, the inverter is provided with an auxiliary circuit comprising a precharged commutation capacitor and at least seventh and eighth alternately conducting unidirectional controllable electric valves that are arranged to connect the capacitor between the neutral or common point of the 3-phase a-c load circuit and either one of the d-c terminals of the bridge.
During each full cycle of steady state operation of a third harmonic inverter, each of the valves in the auxiliary commutation circuit is briefly turned on three separate times. More particularly, the 7th valve is fired at intervals of approximately 120 electrical degrees, and the 8th valve is fired at similar intervals that are staggered with respect to the intervals of the 7th valve, whereby one or the other auxiliary valve is fired every 60 electrical degrees. When an auxiliary. valve is turned on, the commutation capacitor is effectively placed in parallel with one phase of the load circuit and a first one of the two main valves which are then conducting load current. Initially, the capacitor voltage magnitude is higher than the amplitude of the line-to-neutral voltage that is developed across the inductive load, and its polarity is such that the capacitor starts discharging. Consequently current is forced to transfer (commutate) from the first main valve (i.e., the offgoing or relieved valve) to a parallel path including the turned-on auxiliary valve and capacitor. The rate of change of current during commutation will be limited by the load inductance.
After current in the offgoing main valve decreases to zero, the magnitude of capacitor voltage is still sufficient to keep that valve reverse biased for longer than its "turn-off time." As soon as the commutation capacitor is fully discharged, load current begins recharging it with opposite polarity. Once the commutation capacitor is recharged to a voltage magnitude exceeding that of the line-to-neutral load voltage, the next (oncoming) main valve in the bridge is forward biased and can be turned on, whereupon load current commutates from the turned-on auxiliary valve and commutation capacitor to the oncoming main valve. This causes the auxiliary valve to turn off and completes the commutation process. The capacitor is left with voltage of proper polarity and sufficient peak magnitude for successful commutation of the second one of the first-mentioned two conducting main valves when the opposite auxiliary valve is turned on approximately 60 degrees later. It will be apparent that there are six intervals of commutation per cycle, the direction of current in the commutation capacitor during each interval is reversed compared to the preceding interval, and therefore the fundamental frequency of the alternating capacitor current equals the third harmonic component of load frequency.
As is pointed out in the referenced Steigerwald and Lipo paper, one practical application of a current-fed third harmonic auxiliary commutated inverter is in an adjustable speed a-c drive system where the 3-phase star-connected stator windings of a synchronous machine are supplied with variable frequency a-c power by the inverter which needs to be forced commutated in order to start the machine. In such an application, for reasons explained in that paper, a technique of "delayed gating" is used to ensure that at the end of each commutation interval the commutation capacitor has recharged to a sufficiently high level of voltage to guarantee successful commutation during the succeeding interval. According to this technique, the sequential firing signals for the main valves are each delayed, after the oncoming valve is forward biased, while the capacitor continues accumulating and storing electrostatic charge until its voltage attains a threshold level required for extinguishing current in the next offgoing valve. Steigerwald and Lipo suggest that the threshold level can be proportional to the magnitude of source current so that the peak magnitude of capacitor voltage (and hence the commutating ability of the inverter) will desirably track the demands of the load. In other words, the magnitude of capacitor voltage is high when necessary to commutate high current and is relatively low when only light load needs to be commutated. This "adaptive" commutation capacitor voltage technique advantageously reduces commutation time and power losses during the commutation intervals when the magnitude of load current is relatively low.
In his prior art U.S. Pat. No. 4,244,017, Steigerwald discloses and claims a modified third harmonic auxiliary impulse commutated inverter having parallel commutation circuits which allow three different values of commutating capacitance to be actively selected as a function of the magnitude of load current. Assuming that the inverter is supplying a synchronous machine load, current tends to decrease as frequency (i.e., rotor speed) and hence machine back emf increase. By. switching to a commutation capacitor of smaller size when the current magnitude falls below a preset level, the commutation time is desirably shortened at light loads. As a result, the maximum permissible fundamental frequency is increased, and the operating range of the inverter is extended.
A current-fed third harmonic inverter is well suited for supplying variable frequency alternating current to the 3-phase stator windings of a rotatable synchronous machine that is used to start or "crank" a prime mover such as a large internal-combustion engine. In such a system, the rotor of the machine is coupled to a mechanical load comprising the crankshaft of the engine. Initially the output torque of the rotor (and hence the magnitude of current in the stator windings) needs to be relatively high in order to start turning the crankshaft. As the rotor accelerates from rest, less torque (and current) will be required, while the fundamental frequency of load current increases with speed (revolutions per minute). In its cranking mode of operation, the inverter supplies the machine with current of properly varying magnitude and frequency until the engine crankshaft is rotating at a rate that equals or exceeds the minimum speed at which normal running conditions of the engine can be sustained. It should be apparent that the above-mentioned adaptive commutation technique, wherein decreasing current is accompanied by lower commutation voltage and hence shorter commutation intervals, will desirably raise the upper limit of the permissible range of inverter operating frequency.